Call progress tones provide an indication to telephone users of the progress and result of a call being initiated or in process. Different frequencies, various levels, and patterns of tones indicate different conditions, such as whether a dialed number is busy or is out of service, or that a dialed device is being rung. There are many variations of the composition and cadencing pattern of tones involved in call progress indication between geographic regions.
A set of call progress tones which is used for a particular country or geographic location is referred to as a plan. Call progress tone plans have been created by the switching system manufacturer, and are provided with the switching system. In some cases the hardware supplied for the tone generation is unique for each unique region. In other cases it forms part of the software used by the switching system processor, and therefore a full switching system software load is required to be provided to implement a call progress tone plan for a particular geographic region. This requires the switching system to be out of service for the interval of the load, and errors can be introduced during the full software load.
In order to alleviate this problem, for signalling, in one prior art system every possible signal generation software module was provided by the manufacturer to implement a signalling plan, and particular ones to be used were chosen by the telephone company customer and implemented, at the unique location. This has been described in the patent application entitled "Telephone or Data Switching With Variable Protocol Inter-Office Communication", U.S. Ser. No. 494,668, filed Mar. 16th, 1990, and assigned to Mitel Corporation. In that invention object code sub-modules which generate the tone signals to change the inter-office protocol for the system for every possible tone is supplied to the final unique site, in order that the tone plan could be defined on-site from the library of sub-modules.
Call progress tone generators can use the sub-modules to provide some of the tones. However, in order to provide all the variable call progress tones for a switching office, I have found that the provision of sub-modules which provide all possible tone plans only one of which is selected at a given site for the generation of a particular tone plan, is slow and wasteful of resources. Yet it is costly and inefficient to rebuild a complete switching system software load for each unique region, and also very costly to deliver different hardware for the tone generator portion of the systems for each unique region.